Rustfmt removes leading pipe from patterns

```rust
match foo {
    | "foo"
    | "bar"
    => {}
    _ => {}
}
```

gets reformatted to

```rust
match foo {
    "foo"
    | "bar" => {}
    _ => {}
}
```

I can understand that the `=>` on its own line needs an RFC, but the removal of the leading pipe makes that rust language feature essentially unusable.